flashmobs or more delicately, tweetups, is the most useful thing I have seen from twitter.

a location and calendar enhanced twitter client could keep track of tweetups in the near future. im thinking of a box that extends the bottom of a UI like twirl. the box shows the lunch that is happening today. a schedule box maybe. im not sure if the focus is on time, with a schedule chart, or location, with a map.

an entry in the box might look like:
12:30pm &lunch *waterfront @user1 @user2 @user3

schedule items are created using a special tweet style. @=person, #=tag, i suggest two new symbols (could be anything) *=location &=event.

example creation tweet.
&lunch tweetup today at the *waterfront at 12:30pm

lunch and waterfront are not specific enough. perhaps another site could maintain a database of location words that are tied to a specific lat/long or address. and another site would map the location keyword (lunch) to a url for event information.

example attending tweet.
&lunch+ today sounds great. i'll be there.

the + says i'm attending. a - would remove an existing RSVP.